Zeolites are ordered microporous aluminosilicates with well-defined crystal structures. Voids of molecular dimensions allow zeolites to catalyze chemical reactions with unique reactivities and selectivities. Synthesis protocols for encapsulating metal clusters within zeolites can expand the diversity of catalytic chemistries, made possible by the ability of microporous solids to select reactants, transition states, and products based on their molecular size and shape, and to protect active sites from larger species that act as poisons by titrating active sites. General protocols for encapsulating metal clusters within zeolites of different void size and geometry can be used to tailor or select zeolite structures for specific catalytic applications; the methods include ion exchange, incipient wetness and incorporation of metal precursors during synthesis. See for example, Gallezot, P., Post-Synthesis Modification I, 2002, p. 257.
The apertures within small and medium-pore zeolites preclude post-synthetic encapsulation protocols via ion-exchange from aqueous media, which require the migration of solvated metal-oxo oligomers that cannot diffuse through the small apertures in such zeolites. Recently, encapsulation methods that exploit the use of ligand-stabilized metal precursors to prevent the premature precipitation of metal precursors as colloidal oxyhydroxides at the high pH and temperatures required for hydrothermal zeolite crystallization have been developed. These protocols have led to the successful encapsulation of Pt, Pd, Rh, Ir, Re and Ag clusters within LTA and Pt, Pd, Ru and Rh clusters within GIS and SOD. Some zeolites require synthesis temperatures that decompose even ligand-stabilized metal precursors. In such cases, encapsulation is forced by first placing metal clusters within zeolites that form at milder conditions (parent structure) and then subjecting the sample to the conditions that convert this parent zeolite to the intended framework (daughter structure), while preserving encapsulation. These protocols have led to the successful encapsulation of Pt and Ru clusters within ANA.
MFI (ZSM-5) is a medium-pore silica-rich zeolite that typically requires high crystallization temperatures (423-473 K) and pH (>11) for its template-free synthesis. Encapsulation in such materials remains inaccessible via procedures involving direct hydrothermal synthesis using ligand-stabilized metal precursors, as well as post-synthesis exchange, except in the case of monovalent or divalent cations.
It would be of benefit to the industry if more efficient and facile methods of metal encapsulation were available for silica-rich zeolites.